Spark erosion apparatus



Dec. 27, 1960 Filed July 16, 1958 0 all M: ZL 4. M

United States PatentO SPARK EROSION APPARATUS Alfred M. A. Maillet,Versailles, France, assignor to La Soudure Electrique Languepin, Paris,France, a company of France Filed July 16, 1958, Ser. No. 748,903

Claims priority, application France July 19, 1957 7 Claims. (Cl. 219-69)The invention relates to apparatus for the working of electricallyconductive materials by means of spark erosion.

Spark erosion apparatus which operates by the discharge of a device forstoring electric energy is often provided with a charging circuitsupplied with a rectified alternating current. It is, in fact, necessaryto have a current of constant direction available in order to be able tomaintain the respective polarities of electrode and work piece so that aminimum of wear in the electrode may be ensured.

The current in use, Whether single-phase or polyphase, is generallymains current at 50 or 60 c./s. In some appliances analternator-supplied current at a somewhat higher frequency is employedand use is made of special charging circuits designed tolimit the arcs.

It has become customary to measure'the speed of the spark-workingoperations, the so-called working speed, by the amount of matter removedper unit of time from a piece worked in this manner. And up to now ithas proved impossible, with most of the known and most generallyemployed methods, to surpass speeds of 100 to 200 mm. /minute.

The applicant has found that the working speed is limited by phenomenaconnected with the intensity of the charge during certain periods oftime within the operational cycle of the spark generator, and dependentto a large extent on the particular lay-out of the latter.

To begin with, it is well known that erosion must be effected by theaction of sparks, to the exclusion of an arc the action of which isdestructive. Now arcs have a tendency to form when, at the close of adischarge from the energy-storing device, the current delivered acrossthe spark by the source which is then momentarily shortcircuited is ofsufficient strength to prolong the spark.

Also, a too rapid increase of the charge intensity immediately followingthe extinction of the spark creates a tension at the terminals of thestorage device which threatens to lead to premature sparking withoutuseful effect, due to the residual ionization which has not as yetdissipated during the discharge interval and which at any rate tends tofavour the formation of an arc.

In most of the existing spark erosion generators attempts were made toobviate these dangers by keeping the strength of the charge currentbelow a certain value, thus preventing the formation of arcs in thespark gap and guarding against too steep a rise of tension at theterminals of the said storage device after each discharge.

To this purpose a charging resistance, possibly augmented by aninductance, is employed to act on the charging circuit. This, the oldestand most generally adopted solution, does not afford working speedshigher than the above-mentioned speeds in the region of 100 to 200 mm.per minute in industrial application.

Similarly, a limitation of the risks of arcformation was proposed by theinclusion of oneor more capacitors,

to be arranged in series in the charging circuit. This solution, betterthan the previous one since it does not limit the charge intensity whenthe capacitors are discharged, atfords outputs of the order of 500 mm.per minute.

Apparatus constructed on this latter principle nevertheless requires avery accurate regulating system which it is difficult to put intoeitect.

It is known that each spark lifts a few particles of the worked materialby causing the formation of a crater element. The dimension of suchelement is the determining factor for the surface condition of the pieceafter working and is a function of the energy /zCE which has accumulatedin a storage device having a capacity C, at the instant of sparking.With a given disruptive tension E i.e. an electrode-workpiece distancewhich must not be excessive, so as not to reduce erosion accuracy, nortoo small as it would then be impossible to remove the metallicparticles produced by working, it is in practice the capacity whichdetermines erosion accuracy, the latter being the greater, and thesurface condition the better, the smaller the capacity C.

For a given capacity C and disruptive tension E the working speed thenbecomes a function of the frequency of spark recurrence and isconsequently the higher the greater the charging speed for suchcapacitance.

The present invention has for its object a spark erosion apparatus inwhich the charging speed of the energystoring device is carried to theextreme limits of possibility; the conditions achieved in pursuance ofthis result lead moreover to the additional and incidental advantage offavouring the regularity of the sparking operation and of preventing theprocess of discharge between electrode and work piece from assuming thecharacter of an arc discharge.

According to the present invention, apparatus for working electricallyconductive materials by spark erosion comprises a charging circuitconsisting of a source of alternating current connected to the inputterminals of a rectifier network, capacitive storage means connected tothe output terminals of said rectifier network, an electrode spaced fromthe material to be worked and defining therewith a spark gap, and adischarging circuit connecting said spark gap in parallel with saidcapacitive storage means characterized in that for a predetermined value(C) of the capacitance of said capacitive storage means, the totalinductance (L) of said charging circuit and the pulsatance (w) of saidsource of alternating current are chosen so as substantially to satisfythe equation Moreover, when the capacity C is equal or larger than 2microfarads, the total self-inductance L is as low as possible andpreferably comprised between 0.1 and 2.5 millihenrys,

In this case, the self-inductance of the circuit is only theself-inductance of the alternator feeding the circuit, in which case Land C being given, the equation is satisfied by adjusting the r.p.m. ofthe alternator, to obtain the adequate value of w.

The invention is illustrated in and will be further described inconnection with the accompanying drawings which shown by way of example,various embodiments of the invention and in which:

Fig. 1 is a schematic diagram of one form of apparatus according to theinvention.

Fig. 2 illustrates curves characteristic of the variations, as afunction of time, of the current and the voltage 3 in the chargingcircuit of Fig. 1 when the condition Lw=1/Cw is accurately fulfilled.

Fig. 3 shows, in the capacity-frequency plane, a section in the range ofcapacity values which lead to the best conditions for spark working whenusing the apparatus shown in Fig. 1.

Referring to Fig. 1, a motor M drives, by way of a speed changing deviceof any preferred type B, an alternator 1 whose frequency is thusvariable. A full wave bridge rectifier network 2 comprising fourmercury-vapour rectifier valves 3 and 3' on the one hand, and 4 and 4'on the other hand, rectifies both recurrent alternations supplied by thesource I. The valves are connected to the energy-storing device, i.e.,to the spark-working capacitor 5, by means of conductors which maycomprise a selfinductance 6 and a resistance 7, and the workingelectrode 8 acts on the work piece 9 across the disruptive gap 10 whichseparates the two electrodes 9 and 8.

In the following the total self-inductance of the circuit, i.e. thesum-total of the self-inductance of alternator 1 and of that of thecircuit, will be called L, and similarly R will be the total resistanceof the circuit. Moreover, the capacitor which constituted theenergy-storing device, will be assumed to comprise the total capacitanceC of the charging circuit.

Owing to the two rectifier pairs 3--'3' and 44', the alternating source1 charges the capacitor 5 in a known manner and in a constant directionas indicated by the fine and bold arrows. This capacitor is cyclicallydischarged into the circuit comprising the capacitor 5, the electrodes 8and 9 and the gap which separates the .latter.

As mentioned before, experience has shown that if the value of thecapacitance C of the storage device 5 has been chosen so as to obtain adefinite surface condition, a primary prerequisitefor achieving aconsiderable working output consists in providing the charging'circuitwith'a very low reactance, i.e. lower than or equal to a few ohms, andin satisfying, therefore, at least, approximately, the equation byinfluencing these different variables, and more'cspecially the pulsationw. i.e. the source frequency, by varying the speed of the alternator.

In a circuit of this kind the current is interrupted in one half of thebridge rectifier circuit 2 with each alternation, to pass into the otherhalf; with each alternation therefore, the current establishes itself ina transient mode in the corresponding portion of the circuit to feed thespark-working capacitor.

In these circumstances, if the tension of the source be given by e=E coswt, the damping factor cc=R/ZL of the ct arging circuit be assumed verylow in relation to w, and if the initial current and tension be acceptedas negligible, the value of the charge intensity is:

an expression in which Z is the impedance of the circuit. 7

E 2 25 cos wt shows that the application of the factor t to theexpression cos (wt) leads to a flattening of the sinusoidalcharacteristic which makes the intensity correspondingly greater thecloser t is to the origin; this circumstance causes a slowing down inthe rise of the charge intensity following the discharge of the storagecapacitor (or spark-working capacitor), and therefore a slowing down inthe recharging of such capacitor which will then not tend to dischargeprematurely.

Under these conditions (reactance negligible) moreover, the chargingcurrent is practically in phase with the source voltage.

Should the charging operation require more than one currentpulsationaccount being taken of the charge acquired by the workingcapacitor during the first alternation-the intensity of the chargeduring the following alternation or alternations will decrease more andmore.

Charging of the said capacitor will be interrupted as soon as thetension at its terminals attains the value of the disruptive tension E(Fig. 2) at the gap which separates the work piece from the workingelectrode. At this moment, in fact, a spark flashes between these twoelectrodes and the capacitance C is discharged within a total time twhich is far shorter than its charging time t These results areillustrated in Fig. 2. This figure shows by a dotted line (curve I) thevoltage of source 1 rectified by the'valves 33 and 44'; by a solid line(curve II) inetrical. It admits the axis of the abscissae as a tangent-at-the origin, then "grows gradually to attain its maximum afteracertain delay r in relation to the maximum of the source voltage, butcollapses practically together with the latter. Its leading slopetherefore is flattened and prolonged as it grows whereas its trailingslope is straightenedand foreshortened as it descends.

-The-tension acquired by the capacity C (curve III) is also tangentialtothe time'axis at the origin, grows very slowly to begin with, then morequickly, and again more slowly so as to admit again a horizontaltangent, at least approximately, as the charging current collapses(charge 2 In divising the Figure 2 it was assumed (merely for reasons'ofsimplicity) that the tension at-the terminals of capacitor 5 attains thevalue of the disruptive tension E in somewhat less than one A.C. cycleof the source. Durorigins now coincide with the ends of the precedingbranches, and that their amplitudes are smaller. At some time near theend of this second alternation the tension at the terminals of capacitor5 attains at point D the value of the disruptive tension E and a sparkis set up in the discharge circuit.

During discharge the tension E at the terminals of the capacitor 5diminishes according to a curve which corresponds with the'arc DD Withinthe same time the current diminishes in the charging circuit as shown bythe end of the curve II until it reaches a low value. Its strengthbegins to recover towards the end of the discharge following acharacteristic'such as shown by the initial portion of curve II. Thenext-following spark is accompanied by a similar drop in strength, etc.

.It should be noticed that the mutual adjustment of L, C and w forobtaining the minimum value of the'impedance Z is in no way a tuning ofthe circuit providing electrical resonance thereof as a matter of factthis circan cannot'oscillate, because the current is always of the samedirection in that part of the ci cuit comprising the capacitor 5.Moreover, as previously explained, after each alternation of current,said current is always a transient current.

The creation of a charging circuit according to the invention which hasa self-induction of very small value has led to the exclusion of thearcing risk although such circuit, at certain instants, may be traversedby currents of considerable strength.

For instance, a charging circuit has been successfully devised in whichresistance was limited to that of its indispensable elements: source,rectifier, etc.; self-induction in the said circuit was of very lowvalue, i.e. the value of that in the said indispensable devices; thesumtotal of the internal self-inductances, reduced in the course ofexperience practically to that of the alternator 1 employed as a source,had a value, according to experiments, of 0.3-0.8 and 1.2 millihenrys.

It was experimentally ascertained that optimum operation was attained inregard to the lowest values of L when the circuit showed its lowestohmic-resistance values.

This surprising result which is contrary to the teachings of all priorart (according to which additional selfinductive impedances are includedin the charging circuit in order to obviate the danger of arcing) may beexplained in the following way.

In the circuit according to the invention which has a lowself-inductance, the ohmic resistance, and in particular that of therectifiers, is by no means negligible. Therefore the damping factor ofthe circuit has a relatively high value. Owing to this fact, theduration of the spark, especially when the employed working capacitorsare large, is quite considerable. Now the spark gap short-circuits notonly the sparkworking capacitor but also the charging circuit so thatthe energy accumulated in the self-inductance may also discharge intothe spark gap provided that the duration of the said discharge is of thesame order than the duration of the spark i.e. provided that the selfinductance as previously explained is low.

Thus, when the spark terminates, the source will have to re-comrnencecharging the self-inductance together with the spark-working capacitancewhich fact stalls the growth of the charging current and must obviouslyreduce the danger of arcing. This is true particularly in regard tospark-working capacitors equal to, or greater than, 2 microfarads. Giventhe low self-inductance values employed, the inherent circuit resistanceis sufficient, without additional resistances having to be used, forobtaining high values for a, and this offers the advantage of reducingto a minimum the losses caused by the Joule effect in the circuit.

Regarding the size of the damping coefiicient 0:, the precedingtheoretical exposition cannot be considered as absolutely exact. Inparticular, the cycle characteristic for the charging circuit does notcoincide rigidly with the cycle of the source, and the intensity of thecharging current is not rigidly in phase with the rectified voltage ofthe source. However, the characteristic progress of the intensity curvesand particularly their delayed growth at the origin of each currentpulsation, remains true. Moreover, to given values of L and C therecorresponds, rather than a single optimum value of the source frequency,a whole range of values on both sides of such optimum value whichcorresponds to the minimum reactance C being expressed in farads and 0.1L 2.5 milli-henrys (2) or, by introducing the frequency f of the AC.source:

These conditions in their entirety are especially applicable to circuitsin which the capacitance of the energystoring device is greater than 2micro-farads. When this capacitance lies below that figure the conditionwhich aims at an approximation to minimum impedance may be regarded assatisfactory and is then written and the limitation of theself-induction value being of lesser importance in practice.

The conditions set out above and defined for the case of a simplecircuit comprising a supply of rectified alternating current, or anequivalent supply, and a charging circuit having in series:self-inductance,- resistance and capacitance, are sufiicient in that anymore complex circuit comprising several self-inductances, resistances orcapacitances in series or in parallel will allow to ensure aspark-working operation with the same underlying efficiencycharacteristics, provided that it fulfils equivalent conditions asstated in detail below:

(a) approximation, within identical limits of proximity, of thecharging-current resistance in regard to 5);

(b) limitation of the self-inductance L of the charging circuit tovalues which take into account inequalities 2 and 2.

The table below shows the results just set out, for capacitance valuesof capacitor 5 between 5000 microfarads (coarse erosion effect) and 0.01micro-farads (very fine erosion efiect).

Table Capacitance Self-inductance Range of adin in missible sourcemicro-iarads rnilli-henrys frequencies 5,000 0.1 to 2. 5 40- 160 2, 0000.1 to 2.5 40- 240 1,000 0.1 to 2.5 320 500 0. 1 t0 2. 5 440 200 0.1 to2.5 640 100 0.1 to 2. 5 270-1, 080 50 0.1 to 2'5 300-1, 560 20 0.1 to 2.5 620-2, 480 10 0.1 to 2. 5 870-3, 480 5 0.1 to 2. 5 1, 100-4, 400 2 0.1to 2. 5 1, 60043, 400

- 1 1 0.1 w 0. 05 241.0 0.5 VLo 0.02 0. 01

The results of this table are condensed in Fig. 3.

For the values of C entered in the co-ordinate system, those frequencieswhich give the best results are comprised in the shaded Zone between thetwo curves a and b; this zone which occupies only a small part of theplane Cf, confines narrowly the frequencies which give an interestingresult, from among all the possible frequencies.

It has already been mentioned that charging the energystoring device mayterminate during the first half-wave of the charging current whichfollows a discharge, or during one of the subsequent half-waves. Allthings being equal otherwise, the growth of the charging current is themore rapid, during the first half-wave, the longer such half-wave lastsand it is preferable, in order to re-charge the said storage devicerapidly, to employ a source at a frequency as low as possible yetcorresponding to the spark-working capacitance in accordance with theindicated formulae. In practice, therefore, storage devices ofsuccessively decreasing capacity will be put into operation and thisprocedure, according to the invention, will lead to the employment ofcurrent sources at increas ing frequency. As a matter of fact, the givenequation 1 Leo-a may be written Lee L being constant, if the value of Cdecreases the pulsatance should be increased for continuously satisfyingthe equation condition.

These variations, on the one hand of spark-working capacitances which goon diminishing and, on the other hand, of source frequencies which go onincreasing, constitute an essential feature of the present inventionwhich provides means for causing the said variations. These meansconsist for example of an alternator capable of being driven atdifferent speeds which may vary in a continuous manner or by successivesteps of different value, either by means of several alternators ofdifferent speed or by means of frequency changers directly supplied withmains current, etc.

Thus, for example, when proceeding with the spark erosion of awork-piece at a tension of approximately 200 v. with capacitors of 2 1.5and 8 micro-farads in succession, frequencies of 975 and 1490 c./ s.were employed respectively, the output obtained being 950 mmfi/ minutein the first case and 640 mmfi/ minute in the second case.

The capacity of the energy-storing device Whose value is determined, asexplained before, by the particular characteristics of the spark erosionto be carried out may sume successive values to which different sourcefrequency values will correspond. The said capacitance may compriseseveral separate capacitors capable of being connected in series, inparallel, or in series-parallel by means of known switching deviceswhose operation may be linked to that of the devices for altering thespeed of the alternator used as source.

I claim:

1. Spark erosion apparatus for working a conductive material, comprisinga source of alternating current, a full-wave rectifier network the inputof which is connected to said source, capacitive storage means having agiven definite capacity C in farads and two terminals connected to theoutput of said rectifier network, an electrode connected to one of saidterminals spaced from the said material to be worked and definingtherewith a sparkgap, and a connection between the other of saidterminals and said material; the pulsatance w of said alternatingcurrent source and the total inductance (L) in henries of the circuitconstituted by said source, said rectifier net work, said capacitivestorage means and the connections therebetween being such as tosubstantially satisfy the equation 2. Spark erosion apparatus accordingto claim 1 wherein the value of said total inductance (L) is greaterthan 0.1 milli-henry and smaller than 2.5 milli-henries.

3. Spark erosion apparatus for working a conductive material, comprisinga single-phase alternator having an inner self-inductance (L) inhenries, a full-wave rectifier network the input of which is connectedto the output of said alternator through conducting members havingnegligible impedances, capacitive storage means having a capacity C infarads and two terminals connected to the output of said rectifiernetwork through conducting members having negligible impedances, anelectrode connected to one of said terminals and spaced from thematerial to be worked to define therewith a spark-gap, a connectionbetween the other of said terminals and said material, and means foroperating said alternator at a speed corresponding to a pulsatance (w)of the single-phase current so as to substantially satisfy the equationof the capacitive storage means whereby to satisfy the.

equation 1 T w u we 6. Spark erosion apparatus according to claim 1where-- in the value of w is comprised between and nay H1 so as tosatisfy approximately the equation condition 1 talk-TC 7. Spark erosionapparatus according to claim 1, wherein the terminals of said capacitivestorage means are limited to said two terminals and the connection ofsaid capacitive storage means is limited to a parallel connection inrelation both to said rectifier network and to said spark-gap.

References Qited in the file of this patent UNITED STATES PATENTSMironofi Jan. 29, 1957 Bruma et al. Mar. 18, 1958

